1. Field of the Invention
The present invention relates to a thin-film transistor and a method of manufacturing the thin-film transistor. More particularly, the present invention relates to a thin-film transistor having an electroconductive film opposed to (i.e., facing) an active layer and a method of manufacturing the thin-film transistor. The present invention also relates to a TFT substrate and a liquid crystal display device. Further particularly, the present invention relates to a TFT substrate having a plurality of thin-film transistors of the present invention and to a liquid crystal display device having such a TFT substrate.
2. Description of Related Art
In recent years, the development of display apparatuses using liquid crystal display devices is being pursued as a display apparatus for OA apparatus. Among various liquid crystal display devices, active-matrix liquid crystal display devices using as a switching device a thin-film transistor (TFT) which is an active element are advantageous in that reductions in contrast and response speed are limited even in a case where the number of scanning lines is large. For this reason, active-matrix liquid crystal display devices are ordinarily used in high-quality display apparatuses for OA apparatus and in high-definition TV displays. Also, an active-matrix liquid crystal display device used as a light valve in a projection-type display apparatus such as a projector has the advantage of easily producing a large-screen display.
In a case where a liquid crystal display device is used as a light valve in a projection-type display apparatus, it is placed between a light source and an optical system through which light from the light source is projected onto a screen or the like. In this placement, the liquid crystal display device is placed so that the light source is on the counter substrate side of the liquid crystal display device, while the optical system is on the thin-film semiconductor device array substrate (TFT substrate) side. The liquid crystal display device controls, on the basis of screen information, the intensity of light transmitted to the optical system side in light entering from the light source at a comparatively high luminance. More specifically, the liquid crystal display device drives TFTs in a switching manner to control the electric field applied to the liquid crystal layer corresponding to each display element, thereby changing the transmittance of each display element to adjust the intensity of light passing therethrough. After light has passed through the liquid crystal display device, it is projected in an enlargement projection manner through the projection optical system constituted by a lens.
In active-matrix liquid crystal display devices, a layer of a semiconductor such as amorphous silicon or polycrystalline silicon is used as a TFT active layer. When light enters this active layer, a light leak current is generated due to photoexciation to cause a reduction in contrast for example, resulting in deterioration in display performance. In particular, in a case where an active-matrix liquid crystal display device is used as a light valve in a projection-type display apparatus, light enters the liquid crystal display device at a high luminance and the influence of a generated light leak current is large. In this case, not only light from the light source but also light reflected by the projection optical system enters the active layer of the TFT in the liquid crystal display device. Therefore the influence of light leak current is further increased. In recent years, the development of projection-type display apparatuses reduced in size and improved in luminance has been advanced and the luminance of light incident on a liquid crystal display device used as a light valve tends to increase. For this reason, the light leak current problem has become more serious.
As a technique for reducing light leaks, a technique disclosed in Japanese Patent Laid-Open No. 2003-131261 (patent document 1) is known. In this technique, a first light shielding film having a light shielding property is placed at such a position as to face a gate electrode with the active layer interposed therebetween and a second light shielding film having a light absorbing property is placed between the first light shielding film and the active layer to reduce light entering the active layer. In the technique described in patent document 1, the light shielding effect is increased if the distance between the light shielding film and the active layer is reduced. Effective shielding of the active layer from light is thereby achieved, thus effectively reducing a light leak current. However, if the light shielding film is brought close to the active layer, a problem arises that the light shielding film electrically influences the active layer so that a channel is formed in the active layer on the light shielding film side to cause another leak current on the back channel side of the TFT.
As a technique for solving the above-described problem, a technique described in Japanese Utility Model Laid-Open No. 3-042123 (patent document2) is known. In this technique, an insulating film such as a tantalum oxide having a high interfacial level is placed between the active layer and a light shielding film to reduce the electrical influence of the light shielding film on the active layer, thereby inhibiting the formation of a channel on the back channel side of the TFT. In this arrangement, the distance between the active layer and the light shielding film is reduced to reduce a light leak current while preventing a leak current due to the electrical influence of the light shielding film.
In the technique described in patent document 2, an insulating film of a high interfacial level is placed between the active layer and the light shielding film to inhibit the formation of a channel on the front channel side of the TFT. However, if this arrangement is adopted, the channel cannot be easily formed on the back channel side of the TFT even when a potential for turning on the TFT is applied to the gate electrode. There is, therefore, a problem that the drain current when the TFT is on is also reduced, while the leak current is reduced.
In the technique described in patent document 2, there is a need to place a special insulating film as the high-interfacial-level insulating film between the active layer and the light shielding film, which insulating film is different from the insulating film used in conventional TFTs. For manufacture of such a TFT, it is required that a manufacturing process different from a conventional TFT manufacturing process. As a result, the manufacturing process is complicated and the TFT manufacture throughput is reduced.
An object of the present invention is to provide a thin-film transistor which is designed to solve the above-described problems and in which the reduction in the drain current when the TFT is on can be limited while a leak current due to a light shielding film is reduced. Another object of the present invention is to provide a method of manufacturing a thin-film transistor which enables the thin-film transistor of the present invention to be manufactured without requiring a complicated manufacturing process.